(gardening) Weed Management in Landscapes.pdf

W EED M ANAGEMENT IN L ANDSCAPES

Integrated Pest Management for Landscape Professionals

and Home Gardeners

Weed management in landscape

plantings is often made difficult by the

complexity of many plantings: usually

more than one species is planted in the

landscaped area and there is a mix of

annual and perennial ornamentals. The

great variety of ornamental species,

soil types, slopes, and mulches creates

the need for a variety of weed manage-

ment options. There are also consider-

ations regarding public concern about

the use of chemicals to control weeds.

The choice of a specific weed manage-

ment program depends on the weeds

present and the types of turf or orna-

mentals planted in the area. Because of

the many variables, weeds in land-

scape plantings are controlled by a

combination of nonchemical and

chemical methods.

the site. Identify the weed species in

the area, with particular emphasis on

perennial weeds. The best time to

look for winter annual weeds is mid-

to late winter; perennials and sum-

mer annuals are easiest to identify in

mid- to late summer.

weeds and more tolerant of herbi-

cides applied to the site. Hand-

weeding and keeping weeds from

producing seeds in the landscape

will greatly reduce overall weed

populations.

WEED MANAGEMENT

AFTER PLANTING

When developing a weed management

plan for an existing planting or after an

installation is in place, consider the

types of plants present and the weeds

present and their life cycles (annual,

biennial, perennial) (Table 1).

2. Site preparation. The most often over-

looked aspect of a landscape mainte-

nance program is site preparation.

Control existing weeds, especially

perennials, before any grading and

development are started. Glyphosate

(Roundup, etc.) can be used to kill

existing annual and perennial weeds.

Preplant treatment with fumigants

(available to licensed pesticide appli-

cators only) or soil solarization can

be used if time allows; however, 6

weeks are required for solarization

to work and it is most effective when

done during the time of highest sun

radiation—from June to August in

California.

TABLE 1. Common Weeds in

Landscape Plantings.

Annuals

annual bluegrass

clover (black medic and burclover)

common groundsel +

crabgrass (large and smooth) +

little mallow (cheeseweed)

pigweed (redroot and prostrate)

prickly lettuce

purslane

sowthistle

spurge (prostrate and creeping) +

wild barley

wild oat

Biennials

bristly oxtongue +

Perennials

bermudagrass +

creeping woodsorrel +

dandelion

field bindweed +

kikuyugrass

nutsedge (yellow and purple) +

oxalis (creeping woodsorrel and

Bermuda buttercup)

+ especially troublesome

Most landscape plantings include

turfgrass, bedding plants, herbaceous

perennials, shrubs, and trees. Informa-

tion on integrated pest management

for turfgrass can be found in UC IPM

Pest Management Guidelines: Turfgrass

(see “References”). Use this publication

as a practical review and guide to

weed management options suited to

general types of landscape plantings.

3. Define the type of planting. There are

more weed control options if the

planting consists entirely of woody

plants as opposed to herbaceous

annuals or perennial plants, or a

mixture of all three.

WEED MANAGEMENT

BEFORE PLANTING

An integrated approach, utilizing sev-

eral options, is the most economical

and effective means of controlling

weeds. Begin your weed management

plan for landscapes before planting by

following these five basic steps:

4. Don’t introduce weeds. Weeds are

sometimes introduced in the soil

brought to the landscape site either

when amending the soil or in the

potting mix of transplants.

5. Encourage rapid establishment of de-

sired plants. Use the best manage-

ment practices to get the plants

established as quickly as possible so

that they become competitive with

1. Site assessment. Before soil prepara-

tion and when the weeds are visible,

evaluate the soil, mulch, and slope of

P EST N OTES

Publication 7441

University of California

Agriculture and Natural Resources

Revised August 2001

August 2001

Weed Management in Landscapes

Cultivation and Hand-weeding

Cultivation (hoeing) and hand-

weeding selectively remove weeds

from ornamental plantings. These

methods are time-consuming, expen-

sive, and must be repeated frequently

until the plantings become established.

Cultivation can damage ornamentals

with shallow roots, bring weed seeds

to the soil surface, and propagate pe-

rennial weeds. When cultivating, avoid

deep tilling, as this brings buried weed

Weed control options in the landscape

include hand-weeding and cultivation,

mowing, mulching, hot water treat-

ments, and chemical control. All of

these methods are used at one time or

another in landscape maintenance op-

erations (Table 2). After elimination by

hand-pulling, cultivation, or a post-

emergent herbicide application, the

subsequent growth of annual weeds

can be discouraged with mulches and/

or preemergent herbicides.

seeds to the soil surface where they are

more likely to germinate. Perennial

weeds are often spread by cultivation

and should be controlled or removed

by other methods.

Frequent hand-removal of weeds when

they are small and have not yet set

seed will rapidly reduce the number of

annual weeds. If weeds are scattered at

a site, hand-weeding may be the pre-

ferred management method. Hand-

TABLE 2. How to Manage Weeds in Five Types of Landscape Plantings.

Type of planting and comments

Woody Trees and Shrub Beds . Densely shaded plantings

reduce weeds. Preplant weed control is not as critical as in other

types of plantings. It is often necessary to combine treatments for

complete weed control.

Recommendations

Control perennial weeds before planting (although control may be

possible after planting); use geotextile fabrics with a shallow layer

of mulch or use a thick layer of mulch without a geotextile base;

use a preemergent herbicide, if needed, and supplement with spot

applications of postemergent herbicides and/or hand-weeding.

Perennial weeds may be controlled by manual removal, spot

applications of glyphosate or glufosinate, or, in some instances,

dormant-season applications of preemergent herbicides. Escaped

weeds may be controlled manually or with spot applications of

postemergent herbicides.

Woody Ground Cover Beds. Woody ground covers should

exclude most weeds; however, weed encroachment during

establishment is likely.

Control perennial weeds before planting, although perennial

grasses may be selectively controlled after planting with fluazifop

(Fusilade, Ornamec), clethodim (Envoy), or other selective grass

herbicides. Annual weeds may be controlled with mulch plus a

preemergent herbicide, supplemented with some hand-weeding.

Use geotextiles where possible but do not use them where ground

covers are expected to root and spread. After planting, it is difficult

to make spot applications of nonselective herbicides without

injuring desirable plants. Postemergent control of most annual and

perennial grasses is possible.

Annual Flower Beds. A closed canopy will help shade out many

weeds. Periodic cultivations (at 3- to 4-week intervals and

between display rotations) will suppress many weeds.

Control perennial weeds before planting and carefully select flower

species for weed management compatibility. Annual weeds may be

controlled with mulches, preemergent herbicides, frequent

cultivation, and/or hand-weeding. Perennial grasses can be

selectively controlled with clethodim or fluazifop, or other grass-

selective herbicides, but other perennial weeds cannot be

selectively controlled after planting. Geotextiles generally are not

useful because of the short-term nature of the planting. Avoid

nonselective herbicides after planting.

Herbaceous Perennial Beds. Weed management options in

herbaceous perennial beds are similar to those for annual

flowers, except (1) it is more important to eradicate perennial

weeds as there will be no opportunity to cultivate or renovate the

bed for several years; and (2) fewer species are included on

herbicide labels.

Control perennial weeds before planting; use geotextiles where

possible; use mulches with a preemergent herbicide; and

supplement with hand-weeding.

Mixed Plantings of Woody and Herbaceous Plants. Weed

management is complex because of the diversity of species.

Different areas of the bed could receive different treatments. Site

preparation is critical because postplant herbicide choices are

few.

Plant the woody species first; control perennial weeds in the first

two growing seasons, then introduce the herbaceous species.

Plant close together to shade the entire area. Another option may

be to define use-areas within the bed that will receive similar weed

management programs.

August 2001

Weed Management in Landscapes

weeding can be time consuming and

costly but should be included in all

weed management programs to keep

weeds from seeding.

cereals (rice), municipal composts,

and stones.

Black plastic (solid polyethylene) can

be used underneath mulches to im-

prove weed control. It provides excel-

lent control of annual weeds and

suppresses perennial weeds, but lacks

porosity and restricts air and water

movement. For this reason, black plas-

tic may not be the preferred long-term

weed control method in landscape

beds.

Organic mulches include wood chips,

sawdust, yard waste (leaves, clip-

pings, and wood products), and hard-

wood or softwood bark chips or

nuggets. Bark chips are moderate-

sized particles ( 1 ⁄ 5 to 1 ⁄ 2 inch) and have

moderate to good stability, while bark

nuggets are larger in size ( 1 ⁄ 2 to 2 1 ⁄ 2

inches) and have excellent stability

over time. These materials can be used

in landscape beds containing herba-

ceous or woody ornamentals.

Young weeds in open areas also can be

controlled with small flaming units.

Propane burners are available to rap-

idly pass over young weeds to kill

them. A quick pass over the plant is all

that is necessary; do not burn the weed

to the ground. Flaming is more effec-

tive on broadleaf weeds than grasses.

Be careful not to flame over dry veg-

etation and dry wood chips or near

buildings and other flammable materi-

als, and don’t get the flame near de-

sired plants.

Synthetic mulches, which are manu-

factured materials that are called

geotextile or landscape fabrics, have

been developed to replace black plastic

in the landscape. Geotextiles are

porous and allow water and air to pass

through them, overcoming the major

disadvantage of black plastic. Al-

though these materials are relatively

expensive and time-consuming to in-

stall, they become cost-effective if the

planting is to remain in place for 4 or

more years. Geotextiles are used

mainly for long-term weed control in

woody ornamental trees and shrubs.

Geotextiles should not be used where

the area is to be replanted periodically,

such as in annual flower beds or in

areas where the fabric would inhibit

the rooting and spread of ground cov-

ers. Tree and shrub roots can penetrate

the materials and if the material is re-

moved, damage can occur to the

plant’s root system. This might be a

concern if a fabric has been in place

longer than 5 years. At least one

geotextile fabric (BioBarrier) has an

herbicide encapsulated in nodules on

the fabric that reduces root penetration

problems.

The thickness or depth of a mulch

necessary to adequately suppress

weed growth depends on the mulch

type and the weed pressure. The

larger the particle size of the mulch,

the greater the depth required to ex-

clude all light from the soil surface.

Coarse-textured mulches can be ap-

plied up to 4 inches deep and provide

long-term weed control. Fine-textured

mulches pack more tightly and should

only be applied to a depth of about 2

inches. If the mulch is too decom-

posed, it may serve better as a weed

propagation medium rather than a

means of prevention. Plan to periodi-

cally replenish landscape mulches,

regardless of particle size, because of

decomposition, movement, or settling.

If seedlings germinate in mulches, a

light raking, hoeing, or hand-weeding

will remove the young weeds.

The top growth of older weeds can be

controlled by using a string trimmer.

Annual broadleaf weeds are more ef-

fectively controlled than annual

grasses because the growing points of

grasses are usually below ground. Pe-

rennial weeds regrow rapidly after

using a string trimmer. Be careful not

to girdle and kill desirable shrubs and

trees with repeated use of a string

trimmer.

Mowing

Mowing can be used to prevent the

formation and spread of weed seeds

from many broadleaf weeds into culti-

vated areas by cutting off flower heads.

However, weeds that flower lower

than the mowing blade are not con-

trolled. Repeated mowing tends to

favor the establishment of grasses and

low-growing perennial weeds. Mow-

ing of some ground covers can rejuve-

nate them and make them more

competitive against weeds.

Inorganic mulches, which include

both natural and synthetic products,

are generally more expensive and less

widely used in the landscape. Natural

inorganic mulches are stable over time

and include materials such as sand,

gravel, or pebbles. Most of these prod-

ucts are used in public and commer-

cial plantings. If using a rock mulch,

consider placing a landscape fabric

underneath it. The fabric creates a

layer between the mulch and soil,

preventing rock pieces from sinking

into the soil. The fabric prevents soil

from moving above the rock layer,

which would bring weed seed to the

surface.

Placing a landscape fabric under mulch

results in greater weed control than

mulch used alone. There are differ-

ences in the weed-controlling ability

among the geotextiles: fabrics that are

thin, lightweight, or have an open

mesh allow for greater weed penetra-

tion than more closely woven or non-

woven fabrics.

Mulches

A mulch is any material placed on the

soil to cover and protect it. Mulches

suppress annual weeds by limiting

light required for weed establishment.

Many types of landscape mulches are

available. The most common are bark

and other wood products and black

plastic or cloth materials. Other prod-

ucts that are used include paper, yard

compost, hulls from nuts (pecans) or

To install a landscape fabric, you can

plant first and then install the fabric

afterwards using U-shaped nails to peg

it down. After laying the cloth close to

August 2001

Weed Management in Landscapes

the ground, cut an “X” over the plant

and pull it through the cloth. If laying

down a fabric before planting, cut an

“X” through the fabric and dig a plant-

ing hole. Avoid leaving soil from the

planting hole on top of the fabric be-

cause this could put weed seeds above

the material. Fold the “X” back down

to keep the geotextile sheet as continu-

ous as possible. Weeds will grow

through any gap in the landscape fab-

ric, so it is important to overlap pieces

of fabric and tack them down tightly.

Apply a shallow mulch layer (about 1

inch deep) to thoroughly cover the

fabric and prevent photodegradation.

If weeds grow into or through the

geotextile, remove them when they are

small to prevent them from creating

holes in the fabric. Maintain a weed-

free mulch layer on top of the fabric by

hand-weeding or by applying herbi-

cides. Use of a rock mulch above a

landscape fabric can have greater weed

control than fabric plus organic mulch

combinations.

When mulches are too fine, applied too

thickly, or begin to decompose, they

stay wet between rains and allow

weeds to germinate and grow directly

in the mulch. For best weed control,

use a coarse-textured mulch with a low

water-holding capacity. When used

alone, mulches rarely provide 100%

weed control. To improve the level of

weed control, apply preemergent her-

bicides at the same time as the mulch

(see Table 3). Supplemental hand-

weeding or spot spraying may also be

needed.

is not desired such as when line-

marking playing fields, in play-

grounds, around woody plants, for

edging, and for weeds growing along

fence lines. Some brands of equipment

travel slowly (about 2 mile/hour) and

are probably not cost-effective for

weed control along roadsides. Because

these methods employ boiling water or

steam, workers must be adequately

trained in the use of the machines to

prevent severe burns.

Herbicides for

Landscape Plantings

Herbicides have been effectively used

in many types of landscape plantings

and are most often integrated with the

cultural practices discussed above.

Generally, home gardeners should not

need to apply herbicides to existing

landscape plantings. Hand-weeding

and mulching should provide suffi-

cient control and avoid hazards to de-

sirable plants associated with herbicide

use. Many herbicides listed here are for

use by professional landscape pest

managers and are not available to

home gardeners. To determine which

herbicide(s) are in a product, look at

the active ingredients on the label.

Avoid mulches with a pH less than 4

or that have an “off odor” such as am-

monia, vinegar, or rotten egg smell.

These mulches were stored incorrectly

and contain chemical compounds that

may injure plants, especially herba-

ceous plants.

If using a composted mulch, tempera-

tures achieved during the composting

process should have killed most weed

seeds. However, if the compost was

stored uncovered in the open, weed

seeds may have been blown onto the

mulch. Be sure the mulch is not con-

taminated with weed seeds or other

propagules such as nutsedge tubers.

Yellow nutsedge grows through all

geotextiles but some fabrics are better

at suppressing yellow nutsedge than

others (for more information, see Pest

Notes: Nutsedge , listed in “References”).

Preemergent Herbicides. When weeds

have been removed from an area,

preemergent herbicides can then be

applied to prevent the germination or

survival of weed seedlings. Preemer-

gent herbicides must be applied before

the weed seedlings emerge. Examples

of preemergent herbicides include:

DCPA (Dacthal), dithiopyr (Dimen-

sion), isoxaben (Gallery), metolachlor

(Pennant), napropamide (Devrinol),

oryzalin (Surflan, Weed Stopper),

oxadiazon (Ronstar), oxyfluorfen

(Goal), pendimethalin (Pendulum, Pre-

M), and prodiamine (Barricade).

DCPA, dithiopyr, oryzalin, napro-

pamide, pendimethalin, and prodia-

mine control annual grasses and many

broadleaf weeds and can be used

safely around many woody and herba-

ceous ornamentals. Metolachlor has

become popular because it controls

yellow nutsedge as well as most an-

Hot Water or

Steam Treatments

There are several machines currently

available that use hot water or steam to

kill weeds. These machines are most

effective on very young annual weeds

or perennials that have recently

emerged from seeds. The effect is simi-

lar to that of a nonselective, post-

emergent herbicide. Hot water and

steam are not very effective on peren-

nial weeds with established storage

organs, such as rhizomes and bulbs,

nor do they control woody plants. In

general, broadleaf weeds are more

easily controlled by this method than

grasses. The equipment is expensive to

purchase and maintain, so these ma-

chines are not appropriate for home

use. However, commercial landscap-

ers may find them useful in certain

situations where the use of herbicides

Problems with Organic and Natural

Inorganic Mulches. There are several

problems associated with the use of

organic and inorganic mulches. Peren-

nial weeds such as field bindweed and

nutsedges often have sufficient root

reserves to enable them to penetrate

even thick layers of mulches. Some

annual weeds will grow through

mulches, while others may germinate

on top of them as they decompose.

Weeds that are a particular problem

are those that have windborne seeds

such as common groundsel, prickly

lettuce, and common sowthistle. Ap-

plying mulches at depths of greater

than 4 inches may injure plants by

keeping the soil too wet and limiting

oxygen to the plant’s roots. Disease

incidence, such as root or stem rot,

may increase when deep mulches are

maintained.

August 2001

Weed Management in Landscapes

nual grasses. Isoxaben is used for con-

trol of broadleaf weeds.

control existing plants only and do not

give residual weed control. Their pri-

mary function is to control young an-

nual species, but they are also used to

control perennial species. Clethodim

and fluazifop selectively control most

annual and perennial grasses. Glufo-

sinate (Finale), diquat (Reward), and

pelargonic acid (Scythe) are nonselec-

tive, contact herbicides that kill or in-

jure any vegetation they contact. They

kill annual weeds, but only “burn off”

the tops of perennial weeds. Glypho-

sate (Roundup Pro and others) is a

systemic herbicide. It is translocated to

the roots and growing points of ma-

ture, rapidly growing plants and kills

the entire plant. It is effective on most

annual and perennial weeds.

ticles will have little effect on herbicide

activity.

Timing of a preemergent herbicide

application is determined by when the

target weed germinates, or by when

the weed is in the stage that is most

sensitive to the herbicide. In general,

late summer/early fall applications of

preemergent herbicides are used to

control winter annuals, while late win-

ter/early spring applications are used

to control summer annuals and seed-

lings of perennial weeds. If heavy rain-

fall occurs after preemergent herbicide

application or if a short residual prod-

uct was applied, a second preemergent

herbicide application may be needed.

Generally, herbicides degrade faster

under wet, warm conditions than un-

der dry, cool conditions.

Another important factor is the depth

of the mulch. An herbicide applied on

top of a thin mulch may be able to

leach through to where the weed seeds

are germinating, but when applied to

the top of a thick layer of mulch it may

not get down to the zone of weed seed

germination. Products like oxadiazon

(Ronstar) and oxyfluorfen (Goal) that

require a continuous surface layer

must be placed on the soil surface un-

der the mulch. Suggestions for use of

mulch with herbicides are given in

Table 3.

Avoiding Herbicide Injury. Because of

the close proximity of many different

species of plants in the landscape,

herbicide injury may occur, resulting

in visual plant damage. Herbicide in-

jury symptoms vary according to plant

species and the herbicide and can in-

clude yellowing (chlorosis), bleaching,

root stunting, distorted growth, and

the death of leaves. Granular formula-

tions of preemergent herbicides are

less likely to cause injury than spray-

able formulations. Using a granular

formulation reduces the potential for

foliar uptake, but granules of oxadi-

azon (Ronstar) or oxyfluorfen (Goal)

mixtures will injure plants if they col-

lect in the base of leaves or adhere to

Mulch and Herbicide Placement. The

placement of an herbicide in relation to

an organic mulch can affect the herb-

icide’s performance. Additionally, the

characteristics of organic mulches can

affect how herbicides work. A mulch

that primarily consists of fine particles

can reduce the availability of some

herbicides. The finer the organic mate-

rial (compost or manure, compared to

bark), the greater the binding of the

herbicide. Most herbicides are tightly

bound by organic matter, and while

the binding minimizes leaching, it can

also minimize an herbicide’s activity.

Mulch that is made up of coarse par-

No cultivation should occur after an

application of oxyfluorfen; however,

shallow cultivation (1 to 2 inches) will

not harm napropamide, pendimeth-

alin, or oryzalin. Also, soil type and pH

can affect the activity of some herbi-

cides. Use the information contained in

herbicide labels and from your local

county Cooperative Extension office to

determine the tolerance of an ornamen-

tal plant species to a given herbicide.

Match herbicides with weeds present,

and consider using herbicide combina-

tions. Combinations of herbicides in-

crease the spectrum of weeds con-

trolled and provide effective control of

grasses and many broadleaf weeds.

Commonly used combinations include

tank mixes of the materials listed

above or isoxaben/trifluralin (Snap-

shot), oryzalin/benefin (XL), oxyflu-

orfen/oryzalin (Rout), and oxyflu-

orfen/pendimethalin (Ornamental

Herbicide II). Check the label to deter-

mine which ornamental species the

material can safely be used around and

which species of weeds are controlled.

TABLE 3. Suggestions for Placement of Herbicide with an Organic Mulch.

Herbicide

Application

Devrinol (napropamide)

under the mulch

Gallery (isoxaben)

best under the mulch, moderate control

when applied on top of mulch

OH II (pendimethalin plus oxyfluorfen)

works well both under or over mulch

Pennant (metolachlor)

under the mulch

Ronstar (oxadiazon)

over the mulch

Rout (oryzalin plus oxyfluorfen)

works well both under or over mulch

Surflan (oryzalin)

best under the mulch but provides some

control when applied on top of mulch

Surflan plus Gallery

under the mulch but will give a fair

amount of control even when applied on

top of mulch

Postemergent Herbicides. When

weeds escape preemergent herbicides

or geotextile fabrics, postemergent

herbicides can be used to control estab-

lished weeds. Postemergent herbicides

Treflan (trifluralin)

under the mulch

XL (oryzalin/benefin)

under the mulch

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